Table of contents

The method of zero kinetic energy photoelectron spectroscopy (ZEKE PES) has been adapted for the coincident detection of electron pairs with nearly zero kinetic energy in order to study double photoionization in atoms and molecules. As first examples of this new technique, the state selectivity in the double photoionization in argon and the vibrational structure of the lowest doubly charged potential curves in the nitrogen molecule were investigated.

The de-excitation reaction of a ¹D state Kr²⁺ ion by collision with a He atom has been observed in the 10 meV energy region with an injected-ion drift tube mass spectrometer cooled with liquid nitrogen. The rate coefficient for the reaction was determined to be (7+or-2)*10^-11 cm³ s^-1 at the centre-of-mass energy of 13 meV. In the case of the Kr²⁺+Ar system, no de-excitation reaction was observed in this energy region. It seems that these results cannot be explained at all by a crossing mechanism which involves one-electron capture as an intermediate step.

For electron-complex-atom scattering a new expression for the bremsstrahlung amplitude is obtained in the photon energy region above the K-shell ionization potential.

An investigation of gain in a recombining Li-like Al cylindrical plasma and the development of scaling laws on the 3d-4f and 3d-5f transitions is discussed. The work was carried out using a simple model which approximates the hydrodynamic motion to a similarity expansion, thus keeping the run time sufficiently small to enable a large number of configurations of the system to be examined. The laws are formulated both with and without the inclusion of the escape factor in view of the current uncertainty concerning the role of trapping in such systems. Updated versions of the H-like C scaling laws are presented.

Multiphoton detachment spectra of Cl^-, showing excess-photon absorption, have been obtained using short (100 fs) and intense (up to 5*10¹⁶ W m^-2) optical pulses with a wavelength of 620 nm. In the short-pulse regime the shifts in the electron spectrum can be related directly to shifts of the continuum states. In this way, it is shown experimentally that excess-photon detachment can occur in the absence of channel closure.

The authors present the mathematical basis of a method that permits the extraction of information on the dissociation dynamics of three-body reactions in the photoelectron-photoion-photoion coincidence (PEPIPICO) spectra in the case of a two-step mechanism. The linear moments of all three particles are determined in every case. Under the condition that the excitation source is not polarized, a method to determine the angular distribution of the linear moments of the charged two fragments is presented.

Experimental results of the ionization of heavy water vapour in the tunnelling regime, at two different CO₂ laser frequencies, are presented. The ionization mechanism appears to have a similar dependence on laser intensity as that for atoms, although the ionization probability seems to be higher in the case of the molecules in our experiment. It is proposed that vibrational excitation and strong field dressing of the potential energy surfaces of the molecule help in reducing its ionization potential. Numerical fits of ion number versus peak laser intensity for the ionization potential. Numerical fits of ion number versus peak laser intensity for the ionization products D₂O⁺ and (D₂O)₂⁺ using atomic tunnelling theory provide an estimation for the rate probability of multiphoton excitation and strong field dressing prior to ionization.

Coupled-channel impact parameter cross sections are reported for mutual neutralization in He⁺+H^- and Li⁺+H^- collisions for the impact energy range between 0.07 and 25 keV amu^-1. The results are obtained using a one-active-electron model and a 22-state two-centre atomic orbital basis. They are compared with available molecular orbital calculations as well as with experimental data. In a wide range of energies considered, good agreement is obtained with recently measured cross sections. Comparison of present results with neutralization cross sections for H⁺+H^- collisions reveals some differences in the neutralization curves regarding impact velocity, for different projectiles.

The authors report a study on the depopulation mechanism of low Rydberg states. Caesium atoms have been excited to the 9d and 10d levels by absorption of two resonant photons; collisional transfer from these states to the nearby levels has been studied. The related cross sections show strong dependence on both energy defect and angular momentum l.

State-selective charge transfer and target excitation in collisions of He²⁺ ions with Na has been studied by means of far ultraviolet and visible light spectroscopy. Generally, the experimental results confirm the theoretical predictions by Shingal et al. (1987) for electron capture and for the dominant excitation channel, Na(3p). However in the lower half of the impact energy range studied, 2-9 keV amu^-1, discrepancies in energy dependence and absolute magnitude (up to a factor of approximately 6) are found for the Na(4p) and Na(4d) excitation channels.

A merged-beams technique has been used to measure the rate coefficient ( nu sigma ) for radiative recombination (RR) as well as dielectronic recombination (DR) with C-like F³⁺ ions at relative energies up to 74 eV. The RR measurement is compared to a theory based on the hydrogenic approximation with an effective charge Z_eff. The data indicate that Z_eff is larger than the full ionic charge. This is believed to be due to incomplete screening. DR with Delta n=0 core excitations as well as DR with Delta n=1 core excitations is observed. Structures associated with all three terms of the ground-state configuration of F³⁺ are found in our spectra, whereas no sign is seen of the metastable 1s²2s2p³⁵S state.

The R-matrix method of Bartschat and Burke (1987) has been used to calculate total and single differential cross sections for electron impact ionization of Cr I in its ground ⁷S state for impact energies from near threshold to 100 eV. The total cross section for ionization has a peak value of 30.3 a²₀ at an impact energy of 31 eV. For energies above 30 eV, the total cross section for the production of Cr⁺ (3d⁴4s) ⁶D is significantly greater than that for production of Cr⁺ (3d⁵) ⁶S, while for energies below 30 eV the reverse is true. The single differential cross section exhibits pronounced resonance structure for energy losses of the incident electron that are below the threshold for Cr⁺ (3d⁴4s) ⁶D production. These resonances have been analysed in detail.

Superelastic scattering of electrons from laser-excited ¹³⁸Ba (. . . 6s6p ¹P₁) is used to measure the electron impact coherence parameters for excitation of this state. In particular, the variation with scattering angle of the charge cloud asymmetry parameter, P_l⁺, and the alignment angle parameter, gamma , have been determined at two incident electron energies. Comparison with recent theoretical results based on the unitarized distorted wave approximation has been made taking into account any possible distortion in the measurements due to the influence of a finite interaction volume. This comparison reveals good quantitative agreement for the gamma parameter over the range of scattering angles studied. Differences between the observed and predicted P_l⁺ parameters are small for near-forward scattering but become significant at the larger scattering angles studied.

Charged particle potential scattering in both arbitrary scattering and intense monochromatic electromagnetic wavefield is investigated within the framework of relativistic classical theory, treating the wavefield exactly and the scattering potential by perturbation to first order. The effect of strong wavefields on the coherent energy change of the particle in this process is revealed for large (exceed the initial momentum of the particle) momentum transfers of the scattering potential. The rate of the coherent energy exchange of an electron beam with an intense electromagnetic wave due to the stimulated bremsstrahlung process is also obtained, particularly for the high-intensity limit.

The stimulated bremsstrahlung (SB) scattering of electrons on Coulomb centres in the presence of laser radiation of arbitrary intensity is investigated within the scope of the low-frequency approximation. In the relativistic domain this does not restrict the wave frequency in practice (it holds for wave frequencies up to hard X-rays even for the low relativity of electrons). The coefficient of non-linear absorption of high-intensity laser radiation by an electron beam due to SB is obtained.

Second-order distorted-wave calculations are reported for elastic and inelastic 2s, 2p excitation of hydrogen by 54.4, 100, and 200 eV incident positrons. Exact second-order results are compared with other theoretical calculations and with electron scattering results. The effect of ionization channels and positronium formation channels on elastic and inelastic scattering is discussed.